I'm going to jump right in.... IM2 wasnt a great movie. It was entertaining but fell far short of the awesomeness that was Iron Man. A lot of that had to do with the sheer amount of what they were trying to fit into the movie plot wise.... The origin of Whiplash (I tend to think of him as JUST Whiplash), Tonys issues with his father (I'll touch more on that), his poisoning as a result of the Arc Reactor in his chest, Justin Hammer, the beginning romance with Pepper, the Military trying to take his suite, the intro of Black Widow and the Heavy Shield presence and even hints at the famed Demon in a Bottle story line from the comics. In short there was a lot going on. Even with that or probably BECAUSE of all of that, IM2 is THE most important movie in the Marvel Universe. Here's some of the reasons why (this isn't mentioned to be a critique of the film).
1. Tonys daddy issues... We see in IM that Tony very much looks up to his father. His dad's WW2 contributions are well established in the original. In part two we see some of his post war accomplishments. In short Howard Stark is firmly established as being one of the most important figures of the 20th century. We also see that Tony has issues with him. That he struggles to live up to his dad's legacy and that he feels that legacy as a burden. We also see that him and his father had what many of us think to be a typical wealthy, extremely busy parent / child relationship...Howard seems a bit to preoccupied to give Tony the kind of attention he needs and his fathers approval matters greatly to Tony. This plot point receives a bit of resolution towards the end of IM2. But it dosent go away. We meet a young Howard Stark in Captain America the First Avenger and we see his relationship with Steve Rogers. This is a key plot point from both movies because it's shows up yet AGAIN for a 3rd time in the Avengers. There are several exchanges between Cap and Tony ware the subject of Tonys dad and Caps relationship is brought up... Most clearly when Tony says "this is the guy my dad never shutup about?" That tells us that Tony was raised on story's of Cap. Over the course of 3 movies Tonys resentment or jealousy of cap has been established. It's been minor for the most part with the loan exception being the Loki ruled argument all the Avengers have on the Hellicarrier. Why is this important? Because make no mistake, we WILL GET A VERSION OF CIVIL WAR at some point and this will prove to be an important plot point to keep in mind.
2. Every ones biggest complaint... Shields heavy presence and the feeling that fully half of the movie served as a preview for Avengers. This is absolutely true but more then that, it was completely necessary to the wholeUniversemoving forward. They needed to establish the greater, the larger, cinematic universe with this movie. Without the events of IM2 the transition to the Avengers and Thor and even Cap would have been more difficult. All the phase one elements needed to be strung together and pointed at the Avengers movie. This appears to be very similar to what WB/DC is doing with Batman & Superman. It's establishing the foundation of bigger things. Short of Shield having its own movie they needed to be established in some way. IM2 did that. By the time the Avengers came around we were all (and more importantly, none comic fans) firm iliac with Shield. That was necessary for the organization to be portrait as all powerful as it was in Avengers. It established that they were technicaly advanced, that they had their fingers in a lot of pies and that they had been an active secret entity since WW2. Shields heavy presence was essential in establishing the bigger picture.
In terms of importance to the bigger universe Iron Man 2 was without question the most important film to date. It laid a strong future plot line between its two main characters and it laid the foundation for everything that followed. I don't see it as a week movie, I see it for what it is, the foundation that the cinematic universe was built on. I had intended to make this a bit longer but I'm about to go to comic con in Hartford. I hope that you will look back with a bit of different perspective and rematch IM2 because it was an important movie for ware Marvel is now.